1. Field of the Invention
This invention relates generally to a passenger restraint system for a vehicle and, more particularly, to a passenger restraint system for a vehicle incorporating a combination of a low frequency accelerometer and high frequency pressure transducers for sensing a crash event in order to deploy an air bag associated with the vehicle.
2. Discussion of the Related Art
Many vehicles are equipped with passenger restraint systems incorporating an air bag for protecting vehicle passengers in the event of a crash. In order to deploy the air bag at the proper time, it is known in the art to employ specialized sensors and associated circuitry for detecting the crash event so as to release the air bag at the appropriate time. In order to be effective, the passenger restraint system must be able to separate crash events which would require deployment of the air bag, and those crash events in which deployment of the air bag would be unnecessary or undesirable. For example, for a side or rear-end collision, air bag deployment would provide no benefit to the passenger. Further, it would not be desirable to deploy the air bag for low intensity front-end collisions.
The most widely used sensor for detecting crash events for deploying an air bag is an accelerometer. An accelerometer used in this manner is generally a low frequency sensor which detects rapid deceleration in the speed of the vehicle. Accelerometers have been known to be very effective in detecting a front-end impact for appropriately releasing the air bag, and further, have been effective in preventing the air bag from being deployed during side or rear-end impacts when the air bag would be ineffective. Additionally, accelerometers of this type have been shown to be very effective in deploying the air bag at a desired level of impact violence, so as to prevent the air bag from being deployed in low impact situations when it wouldn't be required.
Many known passenger restraint systems offer different modifications to the above described general principle. U.S. Pat. No. 4,842,301 issued to Feldmaier discloses a crash sensing system for detecting a crash event and for deploying an air bag during an event of sufficient magnitude. Specifically, the crash sensing system includes two acoustic sensors positioned near the front of each side rail of the vehicle. The sensors generate a signal in response to acoustic vibrations due to the deformation of metal components during a front-end crash. The generated signal is filtered and passed through an envelope detector such that if the signal exceeds a predetermined reference amplitude for a predetermined time, the air bag will be deployed.
Other related patents include U.S. Pat. Nos. 5,109,341; 5,073,860; 5,065,322; 5,036,467; and 4,975,850. Each of these patents includes a system for deploying an air bag upon detection of a crash event. These systems each include an accelerometer for providing detection of rapid deceleration of the vehicle during the crash event.
In order for the prior art accelerometers to provide a desirable level of sensing, it has been shown to locate one or more accelerometers or comparable sensing devices in the crush zone of the vehicle. In this configuration, the sensing device(s) or accelerometer(s) is (are) separated from the other components of the passenger restraint system, such as the passenger restraint diagnostic unit and air bag firing capacitors. These components are generally located proximate to the air bag. It is thought to be desirable to locate the accelerometer or sensing device in the passenger compartment of the vehicle so that the entire restraint system can be included as a single assembly. This concept enables the cost of the system to be reduced, as well as the life of the system to be increased. Under these circumstances, accelerometers would be isolated from the crush zone.
In the prior art systems, if the accelerometers were located in the passenger compartment, it is likely that, for certain types of crash events, the accelerometer may develop the appropriate impact signal at a time during the crash event which would cause the air bag to be deployed too late. Therefore, it would not be desirable to include the accelerometer of the prior art systems as part of a single assembly within the passenger compartment of the vehicle. It may be possible, in some cases, to improve the impact signal by redesigning the frontal structure and frame rails of the vehicle in question, but the expense to the vehicle manufacturer would be substantial. This also precludes the use of a single-point assembly in many existent vehicle models.
Other types of sensors can be used to provide an indication of a crash event. One type of sensor is a high frequency pressure transducer for sensing vibrations caused by metal being deformed. A high frequency pressure transducer is capable of providing a sufficiently fast crash indication signal while being located within the passenger compartment. Although a high frequency pressure transducer can detect high frequency signals developed during the crash event to give an adequate earlier indication of crash severity while being located in the passenger compartment, a transducer of this type cannot provide adequate directional information of the crash event. Therefore, this type of sensor may deploy the air bag during rear-end or side crash events when the air bag is not required. Consequently, in order to detect high frequency signals as an indication of crash severity, one may give up directional information obtainable from low frequency sensors.
It may also be shown that a crash into a barrier will cause the barrier itself to generate high frequency vibrations without substantially slowing the vehicle. These vibrations could be transmitted through the frame rails of the vehicle to a high frequency sensor located in the passenger compartment of the vehicle. In this case, a sensor depending solely on the high frequency vibrations to make its crash violence determination might be fooled into determining the crash event to be more severe than it actually is.
What is needed then is a crash sensor which is configured within the passenger compartment of a vehicle and can detect high frequency signals indicative of crash intensity and low frequency signals indicative of crash direction and overall vehicle deceleration. It is therefore an object of the present invention to provide such a crash sensor.